《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_brittleness33

CHAPTER 2 2. COMPOSITION, STRUCTURE AND PROPERTIES OF INORGANIC AND ORGANIC The scientific investigation of composition and properties of inorganic glass started in the last century. By comparison, the development of organic glass is just in its early stages. The question of the structure of glass led to a discussion of whether it exists in a microcrystalline or in an amorphous state From the thermodynamic point of view, all condensed substances at zero temperature in equilibrium conditions should be crystal- line. There are, however, also noncrystalline solids in a metastable state. Relaxation times for crystallisation of these solids are extremely long and they therefore remain amorphous in practice. Crystals are rare and their structures are limited in number and can be reduced to only 14 Bravais lattices. The number of possible non-crystalline structural arrangements, however, is infinite. This diversity of the positions of atoms and molecules does not affect their thermodynamic and transport properties. On the whole, disordered structures are macroscopically presented as homogeneous and iso- tropic media. It is generally assumed today that glass belongs to the predominant non- crystalline solids 2.1 GLASS-FORMING INORGANIC MATERIALS The glassy state is known in some elements, notably selenium and tellurium. Sele nium also forms glassy mixtures with phosphorus. There are also some semiconductive glasses of compounds like AS2, Se3 A number of salts may exist as glass. The best known is BeF 2. Complex types of glass have been prepared containing BeF2 together with NaF, KF, LiF, CaF2, Mg F2 and AlF3. Some nitrates(Na, K), sulfates and chlo- rides have been obtained as small glassy droplets by spraying the molten material onto cold plates. More details on such glasses can be found in ref [1]. Most of these types of glass, however, have little technical significance. This is not true of the glassy metals Amorphous alloys or metallic glass containing two or three components such as PdSi FeB, TiNi, NiPB etc., are new materials that have interesting mechanical and magnetic properties that are highly desirable in modern technical applications. They are produced in the form of strips by quenching the melt extremely rapidly [2]. The most important material termed as glass is formed, however, by oxides [1, 3, 4] The central difference between metallic, semiconductive and oxide glasses lies in the relative strengths of their chemical bonds as measured by the energy gap between occupied and unoccupied electronic states. In oxide glasses this gap is more than 5 electron volts, that is, it lies in the vacuum ultraviolet so that such glasses are transpar-

CHAPTER 2 2. COMPOSITION, STRUCTURE AND PROPERTIES OF INORGANIC AND ORGANIC GLASSES The scientific investigation of composition and properties of inorganic glass started in the last century. By comparison, the development of organic glass is just in its early stages. The question of the structure of glass led to a discussion of whether it exists in a microcrystalline or in an amorphous state. From the thermodynamic point of view, all condensed substances at zero temperature in equilibrium conditions should be crystal￾line. There are, however, also noncrystalline solids in a metastable state. Relaxation times for crystallisation of these solids are extremely long and they therefore remain amorphous in practice. Crystals are rare and their structures are limited in number and can be reduced to only 14 Bravais lattices. The number of possible non-crystalline structural arrangements, however, is infinite. This diversity of the positions of atoms and molecules does not affect their thermodynamic and transport properties. On the whole, disordered structures are macroscopically presented as homogeneous and iso￾tropic media. It is generally assumed today that glass belongs to the predominant non￾crystalline solids. 2.1 GLASS-FORMING INORGANIC MATERIALS The glassy state is known in some elements, notably selenium and tellurium. Sele￾nium also forms glassy mixtures with phosphorus. There are also some semiconductive glasses of compounds like As2. Se3 A number of salts may exist as glass. The best known is BeF2. Complex types of glass have been prepared containing BeF2 together with NaF, KF, LiF, CaF2, MgF2 and A1F3. Some nitrates (Na, K), sulfates and chlo￾rides have been obtained as small glassy droplets by spraying the molten material onto cold plates. More details on such glasses can be found in ref. [ 1 ]. Most of these types of glass, however, have little technical significance. This is not true of the glassy metals. Amorphous alloys or metallic glass containing two or three components such as PdSi, FeB, TiNi, NiPB etc., are new materials that have interesting mechanical and magnetic properties that are highly desirable in modern technical applications. They are produced in the form of strips by quenching the melt extremely rapidly [2]. The most important material termed as glass is formed, however, by oxides [1,3,4]. The central difference between metallic, semiconductive and oxide glasses lies in the relative strengths of their chemical bonds as measured by the energy gap between occupied and unoccupied electronic states. In oxide glasses this gap is more than 5 electron volts, that is, it lies in the vacuum ultraviolet so that such glasses are transpar-

ent and colourless, apart from impurities. The semiconductive glasses have energy gaps near 1, 5 ev and are coloured yellow or red, while in metallic glasses the energy gap is Typical and possible glass-forming oxides are listed in table 1 TABLE 1 GLASS-FORMING OXIDES Typical glass formers 02 Bi20 ZrO V Silica is the constituent material in technical glass. Commercial glass is almost exclusively silicate glass Oxides that apparently do not form glass but may be included in glass to obtain special properties such as chemical durability, low electrical conduc tivity, high refractive index and dispersion, increase in hardness and melting point, etc are listed in Table 2. To obtain glass that transmits infrared, some special components such as AS2S3 and TeO2, are used [51 TABLE 2 GLASS-PROPERTY-MODIF YING OXIDES ALO Pb,o Sro Cdo In term of bond strength between the cation and the oxygen all glass formers have values greater than 5 eV and the typical modifiers have lower values in the range of about 2.8eV[6] 2.1.1 CRYSTALLITE THEORY When coolit from the melting point, many materials pass through a ter perature range in which the liquid becomes unstable with respect to one or more crys- talline compounds. An increase in viscosity, however, may partially or completely prevent the discontinuous change into the crystalline phase. Studies of refractive index changes by heat treatment and investigations of other physical properties of glass led Lebedev [1, 4, 7] to the conclusion that glass contains ordered zones of small crystallites

ent and colourless, apart from impurities. The semiconductive glasses have energy gaps near 1,5 eV and are coloured yellow or red, while in metallic glasses the energy gap is zero. Typical and possible glass-forming oxides are listed in Table 1. TABLE 1 GLASS-FORMING OXIDES Typical glass formers: B203 SiO2 P205 As203 GeO2 As203 Sb203 Possible glass formers: Bi203 ZrO2 V205 Silica is the constituent material in technical glass. Commercial glass is almost exclusively silicate glass. Oxides that apparently do not form glass but may be included in glass to obtain special properties such as chemical durability, low electrical conduc￾tivity, high refractive index and dispersion, increase in hardness and melting point, etc. are listed in Table 2. To obtain glass that transmits infrared, some special components such as As2S3 and TeO2, are used [5]. TABLE 2 GLASS-PROPERTY-MODIFYING OXIDES Na20 ZnO A1203 SnO2 K20 BeO La203 TiO2 Pb20 PbO Y203 ThO2 Cs20 MgO In203 CaO CrO SrO BaO CdO In term of bond strength between the cation and the oxygen all glass formers have values greater than 5 eV and the typical modifiers have lower values in the range of about 2.8 eV [6]. 2.1.1 CRYSTALLITE THEORY When cooling down from the melting point, many materials pass through a tem￾perature range in which the liquid becomes unstable with respect to one or more crys￾talline compounds. An increase in viscosity, however, may partially or completely prevent the discontinuous change into the crystalline phase. Studies of refractive index changes by heat treatment and investigations of other physical properties of glass led Lebedev [ 1,4,7] to the conclusion that glass contains ordered zones of small crystallites

In the crystallite hypothesis, it is assumed that glass may contain both amorphous and crystalline zones which are linked by an intermediate formation. These remarkably small crystallites of about 10 A in size consisting of 3-6 atoms are assumed to be of irregular form with distortions in their lattice. Unfortunately, no stringent experimental evidence can be found to support this hypothesis because even X-ray and electron diffraction structural analysis is unable to detect the possible existence of crystals in the range of about 10A. 2.1.2 RANDOM NETWORK THEORY Extensive X-ray structural analyses of glass as well as studies of the melting pro allowed Zachariasen [8]to explain glass as an extended molecular network without ymmetry and periodicity. The glass-forming cations such as Si and B are sur- rounded by oxygen ions arranged in the shape of tetrahedra or triangles. Regarding the oxygen ions, a distinction must be made between bridging and non-bridging ions. In the first case, two polyhedra are linked together over an oxygen ion, and in the second case the oxygen ion belongs only to one polyhedron and has one remaining negative charge In this way, a polymer structure consisting of long chains crosslinked at intervals is produced. The unbalanced negative charge is compensated by low charge and large size cations, e.g. Na, K, Ca, Ba located in the holes between the oxygen polyhedra Substitution of silicon ions in the network by other large charge and small size cations is possible. The network theory was supported by further X-ray investigations by War ren in 1933 and 1937[9a], and in investigations by other scientists. In Fig. 1, a two- dimensional drawing shows the crystalline state of SiO2(a), the glass network of SiO2 (b)and the glass network of a sodium silicate glass(c) Structure of crystallised silica(A), of fused silica(B)and of sodium silicate glass(C) More recent investigations of chalcogenide glasses such as As2Se3 [9b] and also of oxide glass [9b] in transmission electron microscopy suggest that there exist structural domains, large macromolecules or clusters which are, in the case of silica and other oxide glasses, between 60 and 100 A in diameter [9c]. The domain structure in oxide

In the crystallite hypothesis, it is assumed that glass may contain both amorphous and crystalline zones which are linked by an intermediate formation. These remarkably small crystallites of about 10 A in size consisting of 3-6 atoms are assumed to be of irregular form with distortions in their lattice. Unfortunately, no stringent experimental evidence can be found to support this hypothesis because even X-ray and electron￾diffraction structural analysis is unable to detect the possible existence of crystals in the range of about 10/~. 2.1.2 RANDOM NETWORK THEORY Extensive X-ray structural analyses of glass as well as studies of the melting process allowed Zachariasen [8] to explain glass as an extended molecular network without Si 4+ B 3+ symmetry and periodicity. The glass-forming cations such as and are sur￾rounded by oxygen ions arranged in the shape of tetrahedra or triangles. Regarding the oxygen ions, a distinction must be made between bridging and non-bridging ions. In the first case, two polyhedra are linked together over an oxygen ion, and in the second case the oxygen ion belongs only to one polyhedron and has one remaining negative charge. In this way, a polymer structure consisting of long chains crosslinked at intervals is produced. The unbalanced negative charge is compensated by low charge and large size cations, e. g. Na +, K +, Ca ++, Ba ++ located in the holes between the oxygen polyhedra. Substitution of silicon ions in the network by other large charge and small size cations is possible. The network theory was supported by further X-ray investigations by War￾ren in 1933 and 1937 [9a], and in investigations by other scientists. In Fig. 1, a two￾dimensional drawing shows the crystalline state of SiO2 (a), the glass network of SiO2 (b) and the glass network of a sodium silicate glass (c). 9 Si4 § 9 0 2. ~ Na § A B C Fig. 1 Structure of crystallised silica (A), of fused silica (B) and of sodium silicate glass (C). More recent investigations of chalcogenide glasses such as As2Se3 [9b] and also of oxide glass [9b] in transmission electron microscopy suggest that there exist structural domains, large macromolecules or clusters which are, in the case of silica and other oxide glasses, between 60 and 100/~ in diameter [9c]. The domain structure in oxide

glass is difficult to observe because of the possible polymerisation of the domain inter faces by ambient moisture In this sense, glass can be viewed as an assembly of subunits [10] which is not in pposition with the random network theory Zachariasen [8] has carefully distinguished between random orientation on a local level and cluster formation on a lager scale. The idea of continuous large scale random networks is thus merely a considerable oversimplification of his ideas. Today the network theory is generally accepted for ordinary glass. It appears, however, that some complex multicomponent types of glass may also consist to some extent of very small ordered zones in an amorphous network matrix. This is especially true after heat treatment, which can induce phase separation and crystallisation 2.1.3 PHASE SEPARATION DEVITRIFICATION Glassy materials can be considered as frozen-in liquids, which consist, in the case of xidi materials, of polymer chains with branches and cross linkages. With the excep- tion of quartz glass, all types of industrial glass are multicomponent systems. The fact that glass is a multicomponent material leads, however, to the formation of very com plicated structures. These are characterised by the presence of glass-former skeletons of Intensity curves of X-ray scattering of sodium silicate glass in various states according to Valenkov and Porai-Koshits [Il] a)Original glass, b) glass annealed for 2 hours at 420.C various shapes and also by a varied form of microheterogeneity. a variable short-range order in the distribution of ions and atoms exists, however, inside the microregions of the chemical and structural heterogeneities. The microheterogeneous structure of glass was discovered and studied first for two-component glass by Valenkov and Porai- Koshits [11]. They found that the X-ray diffraction pattern of sodium silicate glass depended on the thermal treatment of the sample, as seen in Fig. 2

10 glass is difficult to observe because of the possible polymerisation of the domain inter￾faces by ambient moisture. In this sense, glass can be viewed as an assembly of subunits [10] which is not in opposition with the random network theory. Zachariasen [8] has carefully distinguished between random orientation on a local level and cluster formation on a lager scale. The idea of continuous large scale random networks is thus merely a considerable oversimplification of his ideas. Today the network theory is generally accepted for ordinary glass. It appears, however, that some complex multicomponent types of glass may also consist to some extent of very small ordered zones in an amorphous network matrix. This is especially true after heat treatment, which can induce phase separation and crystallisation. 2.1.3 PHASE SEPARATION, DEVITRIFICATION Glassy materials can be considered as frozen-in liquids, which consist, in the case of oxidic materials, of polymer chains with branches and cross linkages. With the excep￾tion of quartz glass, all types of industrial glass are multicomponent systems. The fact that glass is a multicomponent material leads, however, to the formation of very com￾plicated structures. These are characterised by the presence of glass-former skeletons of a) 200 100, o b) 2OO .8- 100' _.= O. c) lOOO 600 2o0 o ~ .... Fig. 2 Intensity curves of X-ray scattering of sodium silicate glass in various states according to Valenkov and Porai-Koshits [11] a) Original glass, b) glass annealed for 2 hours at 420~ c) glass after devitrification 0,05 0,15 0,25 0,35 sin 19 various shapes and also by a varied form of" microheterogeneity. A variable short-range order in the distribution of ions and atoms exists, however, inside the microregions of" the chemical and structural heterogeneities. The microheterogeneous structure of glass was discovered and studied first for two-component glass by Valenkov and Porai￾Koshits [11]. They found that the X-ray diffraction pattern of sodium silicate glass depended on the thermal treatment of" the sample, as seen in Fig. 2

he Interpretation of the diffraction patterns showed a clear deviation from the Zacharlasen-Warren concept, according to which the Na ions have a random distribu- tion in the holes between the en ions of the disorderly continuous silica network The pattern indicated a micro-heterogeneous structure [12], that consisted of microre- gions with a sodium metasilicate composition embedded in the glassy silica structure Similar results were also obtained with binary borosilicate glass, three-component sodium borosilicate glass [13] and other types of glass Phase separation in certain optically clear types of glass was also indicated in elec- tron-optical investigations. The phase separation that occurs in some glass, however, does not provide evidence for the crystallite theory. In ma nuclei and crystallites can be found which appear as the result of imperfections in the production technology or of the subsequent devitrification process. If, during the working or annealing processes, the glass is held too long in the temperature region in which crystallisation takes place most readily, it will devitrify and be destroyed. De vitrification is the main factor which limits the composition range of practical types of glass. It is an ever-present danger in all glass manufacture and working. The devitrifi- cation in ordinary glass takes place chiefly on the glass surface [14-17]and manifests itself in different ways: from almost indiscernible microcrystals to a fully developed crystallisation. It appears, however, that devitrification does not always start on a sur- face; it seems to be much more dependent on surface pre-treatment 2.1.4 GLASS-FORMING ORGANIC MATERIALS Organic glass or transparent plastics are synthetic solid materials consisting of polymer compounds that are formed mainly by the elements C, H, O and N. The poly- mer macromolecules are obtained by polymerisation, polycondensation or polyaddition reactions between monomers [18] We distinguish here between thermosets, which undergo a destructive chemical change upon application of heat, and thermoplastics, which can be resoftened repeat edly without any change in chemical composition Thermoplastics are generally pre- ferred for optical applications [19, 20]. Like inorganic glass, they have no fixed melting point but rather a softening region. The plastics can be made fluid and shaped by the application of heat and pr Plastic is increasingly used as a substitute for inorganic glass and other materials. It is, however, often necessary to retain the appearance of the substituted material by special surface treatments. High-impact strength organic polymers have a rapidly ex panding market in applications as diverse as ophthalmic lenses, architectural glass, electronic equipment packaging, various shaped form parts in automotive industry and in the form of foils as substrates for various types of thin films

11 The Interpretation of the diffraction patterns showed a clear deviation from the Zacharlasen-Warren concept, according to which the Na + ions have a random distribu￾tion in the holes between the oxygen ions of the disorderly continuous silica network. The pattern indicated a micro-heterogeneous structure [12], that consisted of microre￾gions with a sodium metasilicate composition embedded in the glassy silica structure. Similar results were also obtained with binary borosilicate glass, three-component sodium borosilicate glass [ 13] and other types of glass. Phase separation in certain optically clear types of glass was also indicated in elec￾tron-optical investigations. The phase separation that occurs in some glass, however, does not provide evidence for the crystallite theory. In many types of glass, crystalline nuclei and crystallites can be found which appear as the result of imperfections in the production technology or of the subsequent devitrification process. If, during the working or annealing processes, the glass is held too long in the temperature region in which crystallisation takes place most readily, it will devitrify and be destroyed. De￾vitrification is the main factor which limits the composition range of practical types of glass. It is an ever-present danger in all glass manufacture and working. The devitrifi￾cation in ordinary glass takes place chiefly on the glass surface [ 14-17] and manifests itself in different ways: from almost indiscernible microcrystals to a fully developed crystallisation. It appears, however, that devitrification does not always start on a sur￾face; it seems to be much more dependent on surface pre-treatment. 2.1.4 GLASS-FORMING ORGANIC MATERIALS Organic glass or transparent plastics are synthetic solid materials consisting of polymer compounds that are formed mainly by the elements C, H, O and N. The poly￾mer macromolecules are obtained by polymerisation, polycondensation or polyaddition reactions between monomers [ 18]. We distinguish here between thermosets, which undergo a destructive chemical change upon application of heat, and thermoplastics, which can be resoftened repeat￾edly without any change in chemical composition Thermoplastics are generally pre￾ferred for optical applications [ 19,20].Like inorganic glass, they have no fixed melting point but rather a softening region. The plastics can be made fluid and shaped by the application of heat and pressure. Plastic is increasingly used as a substitute for inorganic glass and other materials. It is, however, often necessary to retain the appearance of the substituted material by special surface treatments. High-impact strength organic polymers have a rapidly ex￾panding market in applications as diverse as ophthalmic lenses, architectural glass, electronic equipment packaging, various shaped form parts in automotive industry and in the form of foils as substrates for various types of thin films

TabLE 3 GLASS-FORMING ORGANIC MATERIALS Refractive index NAS Methylmethacrylate(70% Styrene(30%)Copolymer Polycarbonate 1.586 e-styrene *=depending on Copolymer 2.1.5 CRYSTALLINE AND AMORPHOUS BEHAVIOUR OF POLYMERS Generally the plastic materials can exist in an amorphous, in a crystalline or in a mixed state. The crystalline state is rare because the mobility of the large molecules that is required for the formation of a periodic arrangement is very low. The complex chemical bonds and the predominant homopolar character lead, in contrast to many inorganic materials, mostly to the formation of very complicated structures, e.g. mono- clinic, rhombic or triclinic crystals Many plastics exist in an amorphous state. The growth of macromolecules produces chains and three-dimensional networks. The disordered polymer structure of organic glass is very similar to the network structure of silicate glass. Typical organic glass hows, however, practically no tendency to crystallisation because of steric hindrance by sidegroups or other large substituents. Some plastics exist in a mixed structural type The crystalline areas in the amorphous matrix are often so extended that they can easily be detected by light scattering. In this case, the material cannot be used for optical applications 2.2 THERMAL BEHAVIOUR OF INORGANIC AND ORGANIC GLASSES Vitrification and softening are second-order phase transitions for both inorganic and organic glasses. To obtain a homogeneous melt with inorganic glass, a temperature is required where the viscosity n of the melt is about 10 Poise. The softening point is at n 10 Poise and the working point has a viscosity of n "10" Poise. The temperature T in the transformation interval for solidification corresponds to a viscosity range for glass of between n=10Poise, the annealing point, and n=10 Poise, the strain It was first clearly shown by Bartenev [21] that when the cooling rate is decreased the solidification te are of silicate glass decreases proportionally. This behaviour was confirmed for many types of silicate glass and organic polymers in subsequent papers by various authors [22]and [23]

12 TABLE 3 GLASS-FORMING ORGANIC MATERIALS Type Composition Refractive index Acrylic Polymethyl methacrylate 1.491 Styrene Polystyrene 1.590 NAS Methylmethacrylate (70%) 1.562' Styrene (30%) Copolymer Poly carbonate --- 1.586' CR 39 Allyldiglycolcarbonate 1.490 ABS Acrylonitril-butadiene-styrene Copolymer *=depending on composition. 2.1.5 CRYSTALLINE AND AMORPHOUS BEHAVIOUR OF POLYMERS Generally the plastic materials can exist in an amorphous, in a crystalline or in a mixed state. The crystalline state is rare because the mobility of the large molecules that is required for the formation of a periodic arrangement is very low. The complex chemical bonds and the predominant homeopolar character lead, in contrast to many inorganic materials, mostly to the formation of very complicated structures, e.g. mono￾clinic, rhombic or triclinic crystals. Many plastics exist in an amorphous state. The growth of macromolecules produces chains and three-dimensional networks. The disordered polymer structure of organic glass is very similar to the network structure of silicate glass. Typical organic glass shows, however, practically no tendency to crystallisation because of steric hindrance by sidegroups or other large substituents. Some plastics exist in a mixed structural type. The crystalline areas in the amorphous matrix are often so extended that they can easily be detected by light scattering. In this case, the material cannot be used for optical applications. 2.2 THERMAL BEHAVIOUR OF INORGANIC AND ORGANIC GLASSES Vitrification and softening are second-order phase transitions for both inorganic and organic glasses. To obtain a homogeneous melt with inorganic glass, a temperature is required where the viscosity r I of the melt is about 102 Poise. The softening point is at rl = 107.6 Poise and the working point has a viscosity of r I -104 Poise. The temperature Tg in the transformation interval for solidification corresponds to a viscosity range for glass of between 11 = 1023 Poise, the annealing point, and 1"1 = l0145 Poise, the strain point. It was first clearly shown by Bartenev [21] that when the cooling rate is decreased, the solidification temperature of silicate glass decreases proportionally. This behaviour was confirmed for many types of silicate glass and organic polymers in subsequent papers by various authors [22] and [23]

On the other hand, the softening temperature Tw is a function of the rate of heating This observation is of great practical importance in glass technology because during nnealing and tempering temperature changes may occur at very different speeds. Thus for different processes, differences in the vitrification temperature of up to 50 to 100C may result. It follows from experimental and mathematical treatments [7] that, at a standard glass transition temperature Tg or at a standard softening temperature Tw vitrification or softening occurs if the rate of cooling or heating is equal to 0.2 deg s for inorganic glass and to 0. 1 deg s" for organic polymers Some experimental data obtained from [7] and [ 18] are listed in Table 4. The values for the linear expansion coefficient a were taken near but clearly below the softening TABLE 4 SOFTENING TEMPERATURE AND LINEAR EXPANSION COEFFICIENT OF INORGANIC AND ORGANIC GLASSES Material 0056-0.08 440-480 0.70-1.10 Aluminium silicate glass 582-842 046-065 Soda lime borosilicate glass 708-815 032-0.52 Polystyrene It can be concluded from these investigations that the structure of glass dep its thermal history. Annealing always increases the density of the glass Different pieces of glass each with a different structure will have different softening temperatures Tw when heated at the same rate For technical applications, it is therefore Useful to choose maximum annealing temperatures below a temperature(T_200)oC to prevent unwanted deformations The expansion coefficient is a property of glass that is greatly affected by changes in composition. The linear expansion coefficient a in the glassy state does not, however, depend on the heating rate in the region below the softening point. It is assumed to be constant within this wide temperature range. As can be seen from Table 4, the thermal expansion of the plastics is much higher than that of inorganic glass. Generally, when glass is bonded with other materials having different rates of expansion, temperature hanges create mainly undesirable forces in the two materials. This affects many prop- erties such as adhesion in the case of deposited thin films. In many problems concerning heat transfer, the thermal conductivity n of the mate- als is an important factor. The rate at which heat is transmitted through glass by con- duction depends on size and shape, on the difference in temperature between the two faces and on the composition of the material. Thermal conductivity is commonly ex

13 On the other hand, the softening temperature Tw is a function of the rate of heating. This observation is of great practical importance in glass technology because during annealing and tempering temperature changes may occur at very different speeds. Thus for different processes, differences in the vitrification temperature of up to 50 to 100~ may result. It follows from experimental and mathematical treatments [7] that, at a standard glass transition temperature Tg sT or at a standard softening temperature Tw sT, vitrification or softening occurs if the rate of cooling or heating is equal to 0.2 deg s ~ for inorganic glass and to 0.1 deg s l for organic polymers. Some experimental data obtained from [7] and [18] are listed in Table 4. The values for the linear expansion coefficient ~t were taken near but clearly below the softening temperature. TABLE 4 SOFTENING TEMPERATURE AND LINEAR EXPANSION COEFFICIENT OF INORGANIC AND ORGANIC GLASSES Material T sT ctl0 -5 (~ (~ Fused silica 1580 0.056 - 0.08 Alkali silicate glass 536 - 696 1.15 - 0.96 Lead silicate glass 440 - 480 0.70 - 1.10 Aluminium silicate glass 582 - 842 0.46 - 0.65 Soda lime borosilicate glass 708 - 815 0.32 - 0.52 Acrylic 76 7-9 Polystyrene 72 6-8 It can be concluded from these investigations that the structure of glass depends on its thermal history. Annealing always increases the density of the glass. Different pieces of glass each with a different structure will have different softening temperatures Tw when heated at the same rate. For technical applications, it is therefore useful to choose maximum annealing temperatures below a temperature (Tg200)~ to prevent unwanted deformations. The expansion coefficient is a property of glass that is greatly affected by changes in composition. The linear expansion coefficient (x in the glassy state does not, however, depend on the heating rate in the region below the softening point. It is assumed to be constant within this wide temperature range. As can be seen from Table 4, the thermal expansion of the plastics is much higher than that of inorganic glass. Generally, when glass is bonded with other materials having different rates of expansion, temperature changes create mainly undesirable forces in the two materials. This affects many prop￾erties such as adhesion in the case of deposited thin films. In many problems concerning heat transfer, the thermal conductivity ~, of the mate￾rials is an important factor. The rate at which heat is transmitted through glass by con￾duction depends on size and shape, on the difference in temperature between the two faces and on the composition of the material. Thermal conductivity is commonly ex-

pressed in calories per centimetre per second per degree. Some data are listed in Tab.5 Compared with metals, the values for glass and plastic are low. Radiation is another heat-transfer process. It may be of greater importance than thermal conduction when the temperature is increased to higher ranges. Table 5 also contains some data on the specific heats of different types of glass and plastic [1, 18]. The specific heat cp of glass, which is important in determining its heat capacity, is a nearly additive quantity and can be calculated from the composition by using the factors for the various oxides. The factors and some experimental data are reviewed in refs. [I] and[4 TABLE 5 THERMAL CONDUCTIVITY AND SPECIFIC HEAT OF INORGANIC AND ORGANIC GLASSES (Typical mean values) 100° 100° 315 354 0.11 0.160.20 Soda-lime silicate glass 23 27 020 Soda-lime borosilicate glass 21 26 30 027 Lead silicate glass 17 0.21 Aluminium silicate glass 24 0.19 3.6 0.32 Thermal conductivity: A(cal cms deg)10 Specific heat: Cp(cal g deg) The ability to withstand thermal shock resulting from sudden changes in tempera ture is important for technical applications of glass. The thermal endurance of inorganic glass, studied mainly by Schott and Winkelmann [24](see also refs. [I] and [25]), is a very complex property. The investigations have led to the definition of a coefficient of thermal endurance F: aE in which P is the tensile strength, E is Young s modulus and p is the density It is interesting to know that most types of glass can withstand much greater tem- perature changes when suddenly heated than when rapidly cooled 2.3 MECHANICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES At normal temperatures, glass usually behaves as a solid material. The most ant properties of solids are elasticity, rheology and strength. These properties not only on the molecular mechanisms of the deformation process, but on the

14 pressed in calories per centimetre per second per degree. Some data are listed in Tab. 5. Compared with metals, the values for glass and plastic are low. Radiation is another heat-transfer process. It may be of greater importance than thermal conduction when the temperature is increased to higher ranges. Table 5 also contains some data on the specific heats of different types of glass and plastic [l,18]. The specific heat Cp of glass, which is important in determining its heat capacity, is a nearly additive quantity and can be calculated from the composition by using the factors for the various oxides. The factors and some experimental data are reviewed in refs. [l] and [4]. TABLE 5 THERMAL CONDUCTIVITY AND SPECIFIC HEAT OF INORGANIC AND ORGANIC GLASSES (Typical mean values) XI04 Cp Material - 100 ~ 0~ 100~ - 100 ~ 0~ 100 ~ Fused silicia 28 31.5 35.4 0.11 0.16 0.20 Soda-lime silicate glass 19 23 27 -- 0.20 -- Soda-lime borosilicate glass 21 26 30 -- 0.27 -- Lead silicate glass 11 14 17 -- 0.21 -- Aluminium silicate glass -- 22 24 -- 0.19 -- Acrylic -- 4.7 .... 0.35 -- Polystyrene -- 3.6 .... 0.32 -- Thermal conductivity: ~, (cal cm "1 s "l deg -l) 10 -4. Specific heat: Cp (cal g-1 deg-l). The ability to withstand thermal shock resulting from sudden changes in tempera￾ture is important for technical applications of glass. The thermal endurance of inorganic glass, studied mainly by Schott and Winkelmann [24] (see also refs. [l] and [25]), is a very complex property. The investigations have led to the definition of a coefficient of thermal endurance F: F= P / X (1) ctE ~/ p. Cp in which P is the tensile strength, E is Young's modulus and p is the density. It is interesting to know that most types of glass can withstand much greater tem￾perature changes when suddenly heated than when rapidly cooled. 2.3 MECHANICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES At normal temperatures, glass usually behaves as a solid material. The most impor￾tant properties of solids are elasticity, rheology and strength. These properties depend not only on the molecular mechanisms of the deformation process, but on the viscous

low and on the structural peculiarities of industrial glass samples. For the application of polymeric materials, it is important to know that they possess extraordinarily com- plex mechanical properties. The high deformability, the marked incompressibility and the high sensitivity to changes in temperature are typical Table 6, some mechanical data on various types of glass are listed [7, 18]. For grinding glass, the so-called grinding hardness is important. The value of this for glass is dependent on chemical composition. The silica base used in most inorganic glass compositions is essentially very hard, but additions of other materials to modify, for example, the optical properties in optical glass will reduce the hardness to a greater or lesser extent. The grinding hardness G H. is defined as a quotient: G H. = rate of re oval of standard glass /rate of removal of sample glass. Values are published in the glass catalogues. Unfortunately, there is no exact relationship between hardness and grinding hardnes 2.4 CHEMICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES Although many types of technical silicate glasses are highly resistant to chemical attack, inorganic glass cannot be treated as an inert material. Chemical reactions take place with water, acids, alkali, salt solutions and various vapours, e.g. SO2. Even appa ently chemically resistant glasses may be attacked locally, producing remarkable changes in the composition of their surfaces compared with the bulk. The effect is much stronger with some optical glasses. Generally, however, plastic materials are more resistant than inorganic glass The water attack starts with the diffusion of H into the glass. This is a rapid process at higher temperatures. The water uptake increases with increasing pressure and the glass begins to swell. The quantity of incorporated water usually amounts to a very small percentage of the weight of the sample. Its presence promotes the tendency to crystallisation. As can be seen in Table 7, silicate glasses are more strongly attacked in alkaline solutions than in neutral or acidic solutions because the alkali supplies hy droxyl ions, which react with the silica network. No protective layer forms during the corrosion The attack of acids differs from that of water because the dissolved alkali and basic xide components are neutralised by the acid. In this way, a silica-rich surface layer is formed which reduces the rate of attack with time. When major amounts of soluble oxides are present, which may occur with some types of highly refractive optical glass, the glass will disintegrate. The corrosion resistance is influenced by the glass composi tion. It increases with higher amounts of SiO2 or of Meo, e.g. CaO, MgO, ZnO and Pbo. Addition of even small amounts of Me2O3 impurities such as B2O3 and AlO ease the resistance. Proper tempering of leached silica-rich surface films on techni- cal glass decreases the porosity and increases the stability with regard to chemical attacks[,3,18,25]

15 flow and on the structural peculiarities of industrial glass samples. For the application of polymeric materials, it is important to know that they possess extraordinarily com￾plex mechanical properties. The high deformability, the marked incompressibility and the high sensitivity to changes in temperature are typical. In Table 6, some mechanical data on various types of glass are listed [7,18]. For grinding glass, the so-called grinding hardness is important. The value of this for glass is dependent on chemical composition. The silica base used in most inorganic glass compositions is essentially very hard, but additions of other materials to modify, for example, the optical properties in optical glass will reduce the hardness to a greater or lesser extent. The grinding hardness G.H. is defined as a quotient: G.H. = rate of re￾moval of standard glass / rate of removal of sample glass. Values are published in the glass catalogues. Unfortunately, there is no exact relationship between hardness and grinding hardness. 2.4 CHEMICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES Although many types of technical silicate glasses are highly resistant to chemical attack, inorganic glass cannot be treated as an inert material. Chemical reactions take place with water, acids, alkali, salt solutions and various vapours, e.g. SO2. Even appar￾ently chemically resistant glasses may be attacked locally, producing remarkable changes in the composition of their surfaces compared with the bulk. The effect is much stronger with some optical glasses. Generally, however, plastic materials are more resistant than inorganic glass. The water attack starts with the diffusion of H + into the glass. This is a rapid process at higher temperatures. The water uptake increases with increasing pressure and the glass begins to swell. The quantity of incorporated water usually amounts to a very small percentage of the weight of the sample. Its presence promotes the tendency to crystallisation. As can be seen in Table 7, silicate glasses are more strongly attacked in alkaline solutions than in neutral or acidic solutions because the alkali supplies hy￾droxyl ions, which react with the silica network. No protective layer forms during the corrosion. The attack of acids differs from that of water because the dissolved alkali and basic oxide components are neutralised by the acid. In this way, a silica-rich surface layer is formed which reduces the rate of attack with time. When major amounts of soluble oxides are present, which may occur with some types of highly refractive optical glass, the glass will disintegrate. The corrosion resistance is influenced by the glass composi￾tion. It increases with higher amounts of SiO2 or of MeO, e.g. CaO, MgO, ZnO and PbO. Addition of even small amounts of Me203 impurities such as B203 and AI20 increase the resistance. Proper tempering of leached silica-rich surface films on techni￾cal glass decreases the porosity and increases the stability with regard to chemical attacks [1,3,18,25]

MECHANICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES modulus (HV, kp mm2)extension p mm )(kp mm )(kg mm) Fused silica 7220-8000 244-2.476900-7400 5,4-8.7 Soda-lime borosilicate glass 202-2276100-7310 65 Aluminosilicate glass 2.40-2.765700-6900 1.19-122300 2-8 6-8 11 yrene 300-400 15 2-6 7-10 TABLE CHEMICAL PROPERTIES Material Chemical attack by H2O NaoH (5%)HCI(5%) 100°C.24h or immersed * Fused silica 8x10 4x10 oda lime silicate glass 12x103 1 x 10 65-1.725°C,4h 26.550°C Soda-lime borosilicate glass 8x10 3.6 2x10 24x104 3.5x10 ractically no weight loss 0.3,25°C,24h* Polystyrene Practically no weight loss 0.2,25°C,24h**

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